package licensing

import (
	"bytes"
	"crypto/ed25519"
	"encoding/base64"
	"encoding/json"
	"errors"
	"io"
	"regexp"
	"time"

	"softbox.local/core/internal/canonicaljson"
)

var (
	ErrInvalidLicense     = errors.New("license is invalid")
	ErrDuplicateField     = errors.New("license has duplicate field")
	ErrUnsupportedNumber  = errors.New("license has unsupported number")
	ErrSignatureMissing   = errors.New("license signature is missing")
	ErrSignatureInvalid   = errors.New("license signature is invalid")
	ErrPublicKeyInvalid   = errors.New("license public key is invalid")
	ErrMachineHashInvalid = errors.New("machine hash is invalid")
	ErrMachineMismatch    = errors.New("license machine does not match")
)

var (
	licenseIDPattern   = regexp.MustCompile(`^lic-[a-z0-9][a-z0-9-]{0,59}$`)
	machineHashPattern = regexp.MustCompile(`^[0-9a-f]{64}$`)
	productIDPattern   = regexp.MustCompile(`^[a-z0-9-]+$`)
	policyIDPattern    = regexp.MustCompile(`^[a-z0-9][a-z0-9._:-]{0,127}$`)
	timestampLayout    = "2006-01-02T15:04:05Z"
	licenseFields      = map[string]struct{}{
		"schema_version": {},
		"license_id":     {},
		"machine_hash":   {},
		"products":       {},
		"issued_at":      {},
		"perpetual":      {},
		"update_policy":  {},
		"rebind_policy":  {},
		"signature":      {},
	}
)

// Verifier validates License v1 documents using one copied Ed25519 public key.
type Verifier struct {
	publicKey ed25519.PublicKey
}

// NewVerifier copies and validates a License v1 signing public key.
func NewVerifier(publicKey []byte) (Verifier, error) {
	if len(publicKey) != ed25519.PublicKeySize {
		return Verifier{}, ErrPublicKeyInvalid
	}
	return Verifier{publicKey: append(ed25519.PublicKey(nil), publicKey...)}, nil
}

// Verify validates one License v1 document and binds it to expectedMachineHash.
func (verifier Verifier) Verify(document []byte, expectedMachineHash string) (License, error) {
	if len(verifier.publicKey) != ed25519.PublicKeySize {
		return License{}, ErrPublicKeyInvalid
	}
	if !machineHashPattern.MatchString(expectedMachineHash) {
		return License{}, ErrMachineHashInvalid
	}

	rootValue, err := canonicaljson.Parse(document)
	if err != nil {
		return License{}, licenseCanonicalError(err)
	}
	root, ok := rootValue.(map[string]any)
	if !ok {
		return License{}, ErrInvalidLicense
	}
	signatureValue, exists := root["signature"]
	if !exists {
		return License{}, ErrSignatureMissing
	}
	signatureText, ok := signatureValue.(string)
	if !ok {
		return License{}, ErrSignatureInvalid
	}
	hasExactFields := hasExactLicenseShape(root)
	delete(root, "signature")

	signedPayload, err := canonicaljson.Marshal(root)
	if err != nil {
		return License{}, licenseCanonicalError(err)
	}
	signature, err := decodeCanonicalSignature(signatureText)
	if err != nil || !ed25519.Verify(verifier.publicKey, signedPayload, signature) {
		return License{}, ErrSignatureInvalid
	}
	if !hasExactFields {
		return License{}, ErrInvalidLicense
	}

	wire, err := decodeLicense(document)
	if err != nil {
		return License{}, err
	}
	license, err := validateLicense(wire)
	if err != nil {
		return License{}, err
	}
	if license.MachineHash != expectedMachineHash {
		return License{}, ErrMachineMismatch
	}
	return license, nil
}

func hasExactLicenseShape(root map[string]any) bool {
	if len(root) != len(licenseFields) {
		return false
	}
	for field := range licenseFields {
		if _, exists := root[field]; !exists {
			return false
		}
	}
	if _, ok := root["schema_version"].(json.Number); !ok {
		return false
	}
	if _, ok := root["perpetual"].(bool); !ok {
		return false
	}
	if _, ok := root["products"].([]any); !ok {
		return false
	}
	for _, field := range []string{
		"license_id",
		"machine_hash",
		"issued_at",
		"update_policy",
		"rebind_policy",
		"signature",
	} {
		if _, ok := root[field].(string); !ok {
			return false
		}
	}
	return true
}

type licenseWire struct {
	SchemaVersion int      `json:"schema_version"`
	LicenseID     string   `json:"license_id"`
	MachineHash   string   `json:"machine_hash"`
	Products      []string `json:"products"`
	IssuedAt      string   `json:"issued_at"`
	Perpetual     bool     `json:"perpetual"`
	UpdatePolicy  string   `json:"update_policy"`
	RebindPolicy  string   `json:"rebind_policy"`
	Signature     string   `json:"signature"`
}

func decodeLicense(document []byte) (licenseWire, error) {
	decoder := json.NewDecoder(bytes.NewReader(document))
	decoder.DisallowUnknownFields()
	decoder.UseNumber()
	var wire licenseWire
	if err := decoder.Decode(&wire); err != nil {
		return licenseWire{}, ErrInvalidLicense
	}
	if err := consumeLicenseEOF(decoder); err != nil {
		return licenseWire{}, err
	}
	return wire, nil
}

func consumeLicenseEOF(decoder *json.Decoder) error {
	var trailing any
	if err := decoder.Decode(&trailing); err != io.EOF {
		return ErrInvalidLicense
	}
	return nil
}

func validateLicense(wire licenseWire) (License, error) {
	if wire.SchemaVersion != 1 || !licenseIDPattern.MatchString(wire.LicenseID) ||
		!machineHashPattern.MatchString(wire.MachineHash) || !policyIDPattern.MatchString(wire.UpdatePolicy) ||
		!policyIDPattern.MatchString(wire.RebindPolicy) || wire.Signature == "" {
		return License{}, ErrInvalidLicense
	}
	issuedAt, err := time.Parse(timestampLayout, wire.IssuedAt)
	if err != nil || issuedAt.Format(timestampLayout) != wire.IssuedAt {
		return License{}, ErrInvalidLicense
	}
	if len(wire.Products) == 0 {
		return License{}, ErrInvalidLicense
	}
	products := make([]string, len(wire.Products))
	seenProducts := make(map[string]struct{}, len(wire.Products))
	for index, product := range wire.Products {
		if !productIDPattern.MatchString(product) {
			return License{}, ErrInvalidLicense
		}
		if _, exists := seenProducts[product]; exists {
			return License{}, ErrInvalidLicense
		}
		seenProducts[product] = struct{}{}
		products[index] = product
	}
	return License{
		LicenseID:    wire.LicenseID,
		MachineHash:  wire.MachineHash,
		Products:     products,
		IssuedAt:     issuedAt,
		Perpetual:    wire.Perpetual,
		UpdatePolicy: wire.UpdatePolicy,
		RebindPolicy: wire.RebindPolicy,
	}, nil
}

func decodeCanonicalSignature(value string) ([]byte, error) {
	signature, err := base64.StdEncoding.Strict().DecodeString(value)
	if err != nil || base64.StdEncoding.EncodeToString(signature) != value || len(signature) != ed25519.SignatureSize {
		return nil, ErrSignatureInvalid
	}
	return signature, nil
}

func licenseCanonicalError(err error) error {
	switch {
	case errors.Is(err, canonicaljson.ErrDuplicateField):
		return ErrDuplicateField
	case errors.Is(err, canonicaljson.ErrUnsupportedNumber):
		return ErrUnsupportedNumber
	default:
		return ErrInvalidLicense
	}
}